LEDs are solid light-emitting devices that generate light from semiconductor chips. In the LED, a PN junction is formed in a semiconductor material so that light is generated by recombination of electrons and holes when an external electric field is applied thereto. In this way, electric energy is converted to optical energy directly. Color of the light emitted from the LED may vary with different semiconductor materials.
In 1907, Henry Joseph Round manufactured a first LED. After about a half-century of development, the LEDs have been successfully manufactured from SiC and III-V Group compounds and have wavelengths covering infrared, red, orange, yellow, and green wavelengths. In the mid-1990s, with continuous breach of GaN epitaxial material, p-type doping, and chip structure design, LEDs of blue, white, and ultra-violet light, an example of which is nitride LEDs, have achieved rapid development.
Currently, testing methods and standards of the LEDs are attracting increasing attention. For example, the National Institute of Standards and Technology (NIST) are trying to establish testing methods and standards for testing light-emitting, temperature, and light-attenuation characteristics of the LEDs. The International Commission on Illumination (CIE) issued CIE127-2007 LED testing methods defining the LED light intensity as an average intensity thereof and specifying a uniform testing structure and size of detectors. However, with rapid development of LED technology, the CIE127-2007 methods become insufficient for testing new LED characteristics. In 2008, the Illuminating Engineering Society of North America (IESNA) published “IESNA LM 80-08: Approved Method: Measuring Lumen Maintenance of LED Light Sources,” defining nominal lumen maintenance life of LED light sources. In 2011, the IESNA published “LESNA TM 21-11: Projecting Long Term Lumen Maintenance of LED Light Sources.” China also makes great achievement in semiconductor light-emitting materials, chip technology, and testing of encapsulated products and has issued nine industrial standards concerning semiconductor illumination. Among these standards, “SJ/T11399-2009: Testing Methods for Semiconductor Light-Emitting Diode Chips” and “SJ/T11394-2009: Testing Methods for Semiconductor Light-Emitting Diodes” provide specification for testing methods of electrical, optical, chromatic, radiation, and thermal characteristics as well as sensitivity to static electricity discharge of the LEDs.
Besides being energy-efficient and pollution-free, the LEDs have another advantage of long lifetime. Typically, the LEDs will not become ineffective suddenly. However, light-emitting performance of the LEDs may degrade with lapse of time. Therefore, it is important to improve reliability and lifetime of the LEDs. LED accelerated degradation online in-situ testing is an important testing method of the LED performances. However, current LED testing technology is insufficient for testing the LED performances such as the reliability and lifetime.
The reliability of the LEDs can be tested by the accelerated degradation method. John. A. EDMOND et al. discloses “System and Method for Accelerated Degradation Testing of Semiconductor Devices,” providing a system for testing on-chip semiconductor devices. In this system, pulses of predetermined current amount are applied to the semiconductor devices within a time period and electrical or optical characteristics of the semiconductor devices are measured before, amidst, and after the application of the pulses. The system is suitable for testing single-chip semiconductor devices made from SiC.
Chinese patent application No. 201110435006.8, entitled “System and Method for Carrying out Optical/Electrical/Thermal Aging Comprehensive Testing on Light-Emitting Diodes,” provides a system and method for implementing in-situ optical/electrical/thermal comprehensive testing of one or more LEDs in an environment of accelerated degradation. However, this system and method cannot effectively test the reliability of the LEDs due to the following reasons.
According to this technology, the devices are subjected to degradation under a constant junction temperature implemented by a constant ambient temperature and current. In this manner, it is possible to find weakness of the LEDs and predict lifetime thereof under the constant junction temperature rapidly. However, in order to predict the lifetime of the LEDs under arbitrary junction temperatures, it may be necessary to use one or more such systems to perform the online comprehensive accelerated degradation testing under different constant junction temperatures. This is laborious and expensive as well as inconvenient for rapid prediction of the lifetime of the LEDs.
This technology does not take into account the influence of humidity and ultraviolet radiation on the LEDs. This influence of humidity and ultraviolet radiation may vary under different currents and temperatures. The humidity and ultraviolet radiation should thus also be taken into account in the accelerated stress environment.
Multi-chip integrated LED modules are becoming widely used in semiconductor illumination products, and thus the testing system should be adapted to test such LED modules.